The present invention relates to a technology for manufacturing a semiconductor laser and a technology for manufacturing a semiconductor laser module with the semiconductor laser built therein, and to, for example, a technology effective for application to a technology for manufacturing a semiconductor laser for optical communications.
In a wavelength division multiplexing (WDM) type optical transmission system, a demand for more speeding-up has been made even to transmission equipment with an increase in the amount (traffic) of data to be transmitted. A DFB (Distributed FeedBack) laser with a modulator for 10 Gbps DWDM (Dense Wavelength Division Multiplexing) built therein has been described in, for example, the November 2001 issue xe2x80x9cElectronic Materialsxe2x80x9d by Institute for Industrial Research, P31-P33. A direct modulation DFB-LD (Laser Diode) or the like has been also described in the present reference.
Unexamined Patent Publication No. Hei 7(1995)-135369 has disclosed, as a semiconductor laser used as a light source for optical communications, a semiconductor laser wherein first and second electrodes are disposed on the same surface so as to allow flip-chip packaging, and a stray capacitance of a device is reduced to thereby enable handling of a high-speed operation. The same reference describes that a cutoff frequency f of a semiconductor laser (semiconductor laser device) becomes f=xc2xdxcfx80RC and a reduction in the capacitance C of the device allows high-speed modulation from this equation. The reference also describes that the capacity of a normal semiconductor laser in which two electrodes are opposed with a substrate and respective layers interposed therebetween, is one by adding a junction capacitance Cj formed between an active layer and a clad layer and a stray capacitance Cd developed between the electrodes.
The present reference has described that the semiconductor laser disclosed therein is formed on a semi-insulating substrate, and a minus electrode 12 and a plus electrode 20 are disposed so as to be electrically isolated from each other on the same surface, whereby the stray capacitance developed between the electrodes is greatly reduced.
In the case of a semiconductor laser having an embedded layer on the main surface side of a semiconductor substrate, the end-point management of embedded growth is difficult and the surface of the semiconductor substrate and the surface of the embedded layer are hard to be identical to each other. A buildup of a boundary face also occurs on the embedded-layer side. As a result, it is difficult to connect both a first electrode provided on the surface of the semiconductor substrate and a second electrode provided on the surface of the embedded layer to their corresponding electrode portions of a printed circuit board in a satisfactory state through the use of bonding materials under a so-called junction down state in which the side of a pn junction is set as a mounting surface. Yield degradation and deterioration in packaging reliability are produced.
Therefore, the present inventors have discussed that in a ridge structure in which the surface of the semiconductor laser is made flat, the first and second electrodes are disposed on the surface of the semiconductor laser to carry out flip-chip packaging. Namely, while trenches or grooves exist on both sides of a ridge (stripe) to form the ridge in the ridge structure, the outer surface of each trench is identical in height to the surface of the ridge.
FIGS. 31 through 33 are respectively diagrams related to a semiconductor laser (semiconductor laser device) for high-speed optical communications, which has been discussed prior to the present invention, wherein FIG. 31 is a typical perspective view of the semiconductor laser, FIG. 32 is an enlarged view of a ridge waveguide portion, and FIG. 33 is an equivalent circuit diagram of the semiconductor laser, respectively.
The semiconductor laser 60 has a structure wherein it has a multilayered semiconductor layer including an active layer on a main surface of a semiconductor substrate 61, and an anode electrode 75 and a cathode electrode 76 are respectively provided on the surface thereof and the back surface thereof.
The semiconductor substrate 61 serves as an n-InP substrate 61, for example. A lower SCH (Separate Confinement Heterostructure) layer 62 formed of an n-InGaAlAs layer, an active layer 63 made up of an InGaAlAs layer, an upper SCH layer 64 made up of a p-InGaAlAs layer, a p-InP layer 65 and a p-InGaAs layer 66 are sequentially laminated and formed on the n-InP substrate 61.
The multilayered crystal layer is provided with two grooves or trenches 67 in parallel on its surface. A ridge (stripe) 68 is formed at a portion interposed between the two trenches 67. The trenches 67 are defined by removing, by etching, the p-InGaAs layer 66 corresponding to the top layer of the multilayered crystal layer and the p-InP layer 65 provided therebelow. The upper SCH layer 64 is exposed at the bottom of each trench 67. The ridge 68 has a width of 2 xcexcm and a length of 200 xcexcm.
An insulating film 69 formed of an SiO2 film is provided so as to extend beyond the trenches 67 from both sides of the ridge 68 respectively and extend over the p-InGaAs layers 66 located outside the trenches 67. Namely, only the upper surface of the ridge 68 is exposed without being covered with the insulating film 69. An anode electrode (p electrode) 75, which electrically contacts the p-InGaAs layer 66 for forming the upper surface of the ridge 68, is provided up to an area extending from the ridge 68 to outer edge portions of the trenches 67 on both sides of the ridge 68. The anode electrode 75 is formed with wire bonding portions 75a which extend out so as to become wide on the center side of the ridge 68 and are connectable with wires (see FIG. 31). A cathode electrode (n electrode) 76 is provided on the back surface of the n-In substrate 61.
Incidentally, the active layer 63 takes a multiple quantum well (MQW) structure. Diffraction gratings are provided on the surface of the n-InP substrate 61 along the longitudinal direction of the ridge 68 to thereby configure a distributed feedback semiconductor layer (DFB-LD).
When a predetermined voltage is applied between the anode electrode 75 and the cathode electrode 76 in such a semiconductor laser 60, laser light is emitted from the end of the active layer 63 corresponding to the ridge 68.
A reduction in capacitance is essential for a high-speed modulation laser diode (semiconductor laser). As means for achieving the capacitance reduction, there is (1) a method of reducing electrode areas or (2) a method of making thick a dielectric material between electrodes under a structure wherein the electrodes are opposed to each other with an active layer interposed therebetween. When the anode electrode 75 is further reduced to make each electrode smaller in the semiconductor laser 60 shown in FIG. 31, the wire bonding portions 75a must be further reduced, so that a problem arises in that the wire connecting portions partly extend out from the wire bonding portions 75a, for example, and the reliability (bonding property) of wire connectivity is degraded. This is not preferable. FIG. 17(c) shows a dimensional example of the wire bonding portion 75a employed in the semiconductor laser 60. Since a light-emitting property is impaired, each wire is not bonded onto a ridge portion, i.e., an optical waveguide (resonator) but bonds a position distant from it, i.e., the wire bonding portion 75a. Even where the area of the wire bonding portion 75a is reduced for the purpose of the capacity reduction, the wire bonding portion 75a needs a quadrangular area whose one side is about 80 xcexcm in the case of a wire having a diameter of about 25 xcexcm.
Even in the junction down state in which the semiconductor laser 60 is mounted with being turned upside down, a reduction in the area of each wire bonding portion 75a will degrade the reliability of connection of the semiconductor laser 60 by the bonding materials. The method (2) of making thick the dielectric material between the electrodes is difficult even on a process basis.
FIG. 33(a) is an equivalent circuit diagram of the semiconductor laser 60 as shown even in the enlarged view of FIG. 32. A capacitor C1 and a capacitor C2 exist in parallel with the laser diode LD between the p electrode and the n electrode. The capacitor C1 is a capacitance between the p electrode which interposes the insulating film 69 therein and the upper SCH layer 64, and the capacitor C2 is a capacitance (junction capacitance) of the active layer 63. The resistor R1 is a resistance between the p electrode and the active layer 63, and the resistor R2 is a resistance of the upper SCH 64 (as viewed in the transverse direction). When the resistor R2 is considerably larger than the resistor R1 and the capacitor C2 is greatly larger than the capacitor C1, the equivalent circuit diagram can be represented by the resistor R1 connected in series with the laser diode LD and the capacitor C1 connected in parallel with the laser diode LD as shown in FIG. 33(b).
It is understood that the semiconductor laser 60 shown in FIG. 31 is large in the capacitor C1 and is not suitable as a light source for high-speed optical communications.
An object of the present invention is to provide a low-capacity (i.e., low capacitance) semiconductor laser.
Another object of the present invention is to provide a semiconductor laser having an anode electrode and a cathode electrode on the same surface side and low in capacity.
A further object of the present invention is to provide a low-capacity semiconductor laser having an anode electrode and a cathode electrode on the same surface side and wide in electrode area.
A yet another object of the present invention is to provide a low-capacity semiconductor laser module.
A still further object of the present invention is to provide a semiconductor laser module high in the reliability of packaging of a semiconductor laser and low in capacity.
The above of the present invention, and other objects and novel features thereof will become apparent from the description of the present specification and the accompanying drawings.
Summaries of typical ones of the inventions disclosed in the present application will be described in brief as follows:
(1) A semiconductor laser comprises a semiconductor substrate, a multilayered growth layer which is formed on the main surface side of the semiconductor substrate in multilayer form and which is electrically isolated from the semiconductor substrate and includes, in a middle layer, an active layer interposed between a semiconductor layer of first conductivity type corresponding to a lower layer and a semiconductor layer of second conductivity type corresponding to an upper layer, a ridge formed between two trenches, which is provided on the surface of the multilayered crystal layer without reaching the active layer, a first isolation trench which is provided outside the trench located on one side of the ridge along the ridge and reaches a semiconductor layer below the active layer, a second isolation trench which is provided outside the trench located on the other one side of the ridge and reaches a surface layer of the semiconductor substrate through the multilayered growth layer, an insulating film which covers the respective surfaces of the trenches, the first isolation trench, the second isolation trench, the multilayered growth layer between the first isolation trench and the second isolation trench, the multilayered growth layer outside the first isolation trench, and the multilayered growth layer outside the second isolation trench, except for the upper surface of the ridge, a second electrode whose part is provided on the semiconductor layer of second conductivity type on the upper surface of the ridge and which is provided on the insulating film located on the multilayered growth layer outside the second isolation trench beyond the second isolation trench as viewed from the upper surface of the ridge, and a second electrode which deviates from above the ridge and is formed on the insulating film on the first isolation trench side and whose part contacts the first conductivity type semiconductor layer located below the active layer.
The semiconductor layer corresponding to the lowest or bottom layer of the multilayered growth layer is Fe-implanted so as to electrically isolate the semiconductor layer above such a semiconductor layer from the semiconductor substrate, thereby resulting in electrically high resistance. The first electrode comprises a flat portion formed on the insulating film located on the multilayered growth layer outside the first isolation trench, and a contact portion which extends in connection with the flat portion and is provided in the flat portion area so as to reach the layer located below the active layer. The length of the active layer extending between the first isolation trench and the second isolation trench is approximately 40 xcexcm.
A semiconductor laser module with such a semiconductor laser built therein comprises a package having a plurality of external electrode terminals, a support substrate encapsulated in the package, a semiconductor laser fixed onto the support substrate and encapsulated in the package, connecting means which electrically connect respective electrodes of the semiconductor laser and the external electrode terminals within the package, and an optical fiber which extends over the inside and outside a case for the package and has an inner end that faces an outgoing surface of the semiconductor laser so as to take in laser light emitted from the semiconductor laser. The support substrate is formed with predetermined wiring patterns on an upper surface thereof. The semiconductor laser is flip-chip packaged to the support substrate in the junction down state. The package comprises a box-shaped package body formed of an insulating resin, and a cap for blocking the package body. The external electrode terminals extend over the inside and outside the package body, and the support substrate is fixed onto some of the external electrode terminals, and wires of the support substrate and inner end portions of the external electrode terminals are connected by conductive wires.
In the semiconductor laser as described above, a portion at which the capacitance is formed, corresponds to a portion that faces a narrow area between a first isolation trench and a second isolation trench. It does not depend on the areas of first and second electrodes provided on a main surface of the semiconductor laser. It is therefore possible to provide a semiconductor laser and a semiconductor laser module, which are capable of achieving a reduction in capacity and performing high-speed modulation.